Field of the Invention
The present invention relates to a white light emitting device and a manufacturing method thereof. More particularly, the present invention relates to a white light emitting device and a manufacturing method thereof configured to embody a white light emitting device by having organic phosphors dissolved, inorganic phosphors-dispersed high molecular resin coated on a light emitting diode, thereby accomplishing an excellent luminous efficiency and color coordinate without creating a compatibility problem with coating resin.
Description of the Prior Art
In general, a light emitting device (hereinafter generally referred to as LED) has a small size, high efficiency, can emit bright color, and as a semiconductor element, is not vulnerable to breakage. Also, it has excellent initial driving characteristics and vibration resistance. Furthermore, it can withstand repeated ON/OFF cycling. Consequently, the LEDs have been widely used as various types of indicators and light sources.
Recently, LEDs of red, green and blue colors with high luminance and high efficiency have been developed, and these LEDs have been used in manufacturing large size LED displays. Such LED displays can operate at low power, have light weight, and long service life. In order to use LEDs to get white light, since LEDs have monochromatic peak wavelength, three LEDs of red, green and blue, each arranged adjacent to each other, have been used in combination to diffuse and blend their colors into white.
In addition, recently, efforts have been made to create individual LEDs which emit white light. However, hue and luminance of each LED are irregular to make it difficult to obtain a desired white light.
Another problem is that, because respective red, green and blue LEDs for creating the white light are manufactured with different materials, a driving circuit becomes complicated due to different driving voltages of respective LEDs.
There are other problems in that because, each suitable LED is typically constructed of a semiconductor epistructure light emitting element set in a cup on the tip of a lead frame, temperature characteristics thereof are different and hues are changed with respect to environment, it is difficult to uniformly mix the lights created by each LED and there are many cases of created high intensity spots. In other words, it should be admitted as a fact that it is difficult to obtain a satisfactory result of creating a white light through mixture of three colors.
In order to address the afore-mentioned problems, there is used a method of a light emitting element for emitting a particular wavelength being absorbed by a phosphor contained in a resin molding that covers the light emitting element. The said method is to create a white light by causing the phosphor to absorb lights emitted by the LED and to emit a light of different wavelength.
In other words, the light emitting element emits blue light and the phosphor absorbs the emitted blue light and emits yellow light. The combination of yellow phosphor light and blue light emitting element makes it possible to manufacture an LED that can emit white light.
Furthermore, a system is employed where an LED of ultraviolet band is used and three different color-emitting phosphors (e.g., red, green and blue) are blended to induce a light emission from an entire visible light region, thereby enabling to obtain white light.
However, if only an inorganic yellow phosphor system (“YAG: Ce”) is used, a halo effect may be produced by wavelength separation phenomenon caused by a narrow peak of blue wavelength region and a wide peak of yellow wavelength, making it difficult to create white light. Most of the hitherto known methods employ the inorganic phosphor system.
FIG. 1 is a schematic drawing of inorganic phosphor particle-dispersed resin which is coated on an LED for creating a light emitting element according to the prior art, where the LED is conventionally bonded to a reflector plate of a lead frame and packaged.
At this time, if an LED (10) emitting blue light is set in a cup-shaped reflector plate (21) on the tip of a lead frame (20), white light can be created by the reflector plate (21) being coated with inorganic phosphor particle (31)-dispersed resin (30) for encompassing the LED (10). In other words, the blue light emitted from the LED (10) is discharged outside as white light via the yellow inorganic phosphor particles (31). The inorganic phosphor uses polymeric dispersion system such that it is inevitable to suffer from disadvantages of luminance loss caused by scatter with regard to dispersion of particles and absorption of molecules.
Another disadvantage is that surface treatment is essential for preventing cohesion of particles and efficiency decreases due to clod of particles as time goes by.
Referring again to FIG. 1, if there is no uniform dispersion, no uniform luminance can be expected, and a serious problem occurs as the degree of phosphor content is enlarged, thereby resulting in creation of color spot and constituting a factor of causing a decreased efficiency of elements in the long run.
FIG. 2 is a fabricating process flow chart of a white LED according to the prior art, which is disclosed in Korea laid open Patent No. 2003-31061.
First of all, a lead frame having a plurality of terminals is packaged with one or more LED chips for electrical contact. (S10). The metal terminals of the lead frame and the LED chips are electrically connected via a bond wire (S20). Successively, granular inorganic phosphor and liquid organic phosphor are blended (S30), and the blended phosphors are mixed with resin composition (S40). The mixed resin is coated on the LED chips and hermetically sealed (S50). Next, the resin is hardened (S60).
The light emitted from the said LED passes a light converting layer composed of the mixed resin composition and is converted to white light and discharged outside to become a LED emitting white light.
However, various problematics may arise in the prior art thus described because the organic phosphors are dissolved in separate liquid resin to form liquid organic phosphors.
First of all, there may be a problem in compatibility with the coating resin, and because of difference in solubility, and if dispersed in the coating resin, a crystallinity phenomenon occurs, making it difficult to obtain a uniform dispersion system. Furthermore, after the coating, residual solution or liquid resins each having a different property may result in creation of air bubbles or cracks during the hardening process.
If an organic phosphor of low solubility is employed, an amount of solvent is increased to result in a low viscosity of coating resin such that it is difficult to unify the coated amount to an LED per lot, and the coated amount varies in response to the degree of the solvent being volatilized, making it difficult to obtain a uniform luminance effect and color coordinate.
Furthermore, the addition of phosphors to an LED results in a relatively costly manufacturing process that yields separate solution process and a mixing process.
In addition, the said organic phosphors which are not composed of pure organic phosphors may contain foreign objects to reduce the luminance efficiency.
Consequently, the method of mixing the liquid organic phosphors with separate coating resin as disclosed in the prior art of Korean Patent Laid-open Publication No. 2003-0031061 is deemed to be inadequate and has created the aforementioned problems.